Getting The Most From Your Audio Processor

On-Air Audio ProcessingGetting the most from your broadcast audio processor.

Lukas Hurwitz - Sales & Marketing Manager, Inovonics

Audio processing... no technical aspect of radio broadcasting is so subjective in its perception, or elicits such an emotional response, as on-air broadcast audio processing. Everyone has an opinion on what sounds good, and with such a wide range of hardware manufacturers making products to get the job done, is there really any way to know whether youʼre getting the most out of your particular piece of hardware regardless of opinion or product?

Thankfully the answer is yes! But to get the most out of a given processor we need to first make sure we understand the basic building blocks of the modern multi-band broadcast processor, wrap our heads around loudness as itʼs perceived by the human ear, understand the particular station format, and finally how to get help from what the processor displays. For the purposes of this article, Iʼll be using examples from Inovonicsʼ own DAVID IV FM/HD-Radio Audio Processor, but since most modern processors operate in much the same way, the concepts easily translate to your processor of choice.

UNDERSTAND THE BASIC BUILDING BLOCKS

Modern broadcast audio processors generally treat program material in the same way: feeding audio through a series of processing “blocks”, treating dynamics at a rate from slow (AGC), to fast (Limiting), to virtually instant (Clipping). Splitting up the signal path into these stages provides users the ability to isolate each stage and tweak one at a time if necessary. But where do we start? AGC, Compression, Limiting or Clipping?

If we step back and think about starting from scratch on any type of creative project, whether itʼs painting, carving a sculpture, or building a house, weʼd start big and work through the details as we come to the end, right? In the case of multi-stage dynamics processing that means working from slow to fast, start with getting AGC in the ballpark, followed by Compression, Limiting, and finally Clipping.

Inovonics Inc. November 2014

Side note: A variety of additional audio effects like Stereo Field or Bass Enhancements can often be found inserted between these blocks in the signal path but weʼll skip over those in this post as theyʼre best treated as “icing-on-the-cake” once the core blocks are dialed in properly.

Additionally, there will almost always be some going back and re-adjusting after all the elements are in range, but if we stick to working from slow dynamics processing to fast, weʼll save huge amounts of time and get better results with more confidence.

LOUDNESS & CONTRAST

Take a look at the image above and focus on the orange dots inside of each cluster of blue dots. Which orange dot is bigger? The one on the right? Wrong. Take a ruler to your screen and compare them, theyʼre exactly the same size, but to the human eye they look different because we see in terms of relative size and context. Itʼs called an Ebbinghaus illusion, and when it comes to perceived loudness, our hearing mechanism works in a similar way. Clap your hands in a library and it sounds a whole lot louder than if you do the same in a crowded bar because itʼs all about relative context of the audio levels around you. Thatʼs why itʼs vitally important to try and maintain a healthy range of dynamics (the difference between loud and quiet elements) in our sound even if consistently peaking at 100% Modulation. Maxing out at 100% Modulation all the time with very little dynamic movement means listeners will actually perceive a muffled and quieter sound over time. Not good! So whether weʼre competing with other stations in our market or just wanting to hype our audio, we should always try to maintain as much dynamic contrast as possible, period.

AGC (Automatic Gain Control)

The first processing stage in a broadcast audio processor is almost always wideband AGC, and since itʼs essentially replicating what a real live “hand on the fader” would do, it helps to think about the settings in relation to how a good board operator would respond to gradual changes in audio levels. That means making adjustments for practical reasons (compensating for non-normalized audio or remote feeds) and in a way thatʼs appropriate to a particular format.


But before we start making adjustments to parameters like Makeup Gain, Rate (Speed), or Window size, we should always ensure enough signal level is going into the AGC stage. All AGC stages have a gate function which prevents the AGC action from engaging if the signal level is simply too low. Use your front panel metering (see image above) to make sure the input level, using a sample of typical program material, is enough not only to make the AGC engage, but float somewhere around 0dB. Failure to do this can result in insufficient levels coming out of the AGC stage to feed the subsequent Compression, Limiting and Clipping stages. In other words, the rest of the processing stages will get starved of audio to do their jobs properly.

So now how do we pick the right settings for a particular format? The best way is to first think about the sonic characteristics of your formatʼs typical program material. Only from that information can we know whether our settings are appropriate and best showcase our format. As working examples, letʼs consider two drastically different formats: Talk and Electronic Dance,

A typical Talk format has a wide range of levels that change rapidly and itʼs important to try to sound as natural and transparent as possible (itʼs the human voice after all). That means a quick hand on the fader (Rate) but not so much volume compensation (make-up Gain) as to accidentally pull up background noise during silences. Also, since we want to sound as transparent as possible, dialing in a large Window value is appropriate to establish a wide audio “sweet spot”. That way the AGCʼs Rate, and make-up Gain only start working when audio strays outside the Window.

Conversely, an Electronic Dance format has as relatively narrow range of dynamic levels. Most material is already heavily processed and normalized so levels between tracks donʼt change quickly, and the goal is to be as consistent from track to track as possible (think about replicating a live club vibe). With that in mind we can afford to go with a slow to moderate Rate and a much higher Makeup Gain setting. In addition, since the track levels should be dead-on consistent at all times, we should dial in a really narrow Window.

COMPRESSION & EQ

When referring to compression in a broadcast audio processor we are of course talking about multi-band compression. The main reason why multi-band compression is used over basic single band compression is that it allows us to even-out the spectral balance (Bass, Mids, & Highs) between varying material


produced and mastered differently. The other reason is that by dividing up the audio spectrum into bands, we can drive the compressor harder because transient energy from one band wonʼt cause ducking in another. In other words, a big boomy bass drum wonʼt squash sustained vocal because they are separated by crossover filters.

EQ (Equalization) in a broadcast processor has dual uses. The first being to help shape a unique “Audio Signature” by boosting or cutting select frequencies. Second, to help compensate for the standard 15kHz FM roll off that can cause off-air audio to sound dull compared to CDʼs and digital formats where audio is flat out to 20kHz. The example settings below show a typical EQ curve for a Talk format where everythings cut back but the vocal frequencies, and a Dance format where frequencies accentuating the drums, bass, and percussion are boosted since thatʼs where the majority of the sonic energy is.

Getting loudness and density through multi-band compression is primarily achieved by adjusting the drive into the compressor: the more drive, the more bumps in dynamics and transients are going to get smoothed out, but the less dynamic range we end up with. So it can be helpful to think about compression as “smoothness” and how relevant that audio quality is to a format (say “hello” Smooth Jazz). Compression accomplishes a very different kind of loudness compared to a Peak Limiter, and weʼll get into striking a balance between compression and limiting shortly. But first letʼs look at the Peak Limiter.

PEAK LIMITING

The original purpose of a peak limiter was to literally limit peaks in the audio, or to protect the transmitter from accidental overshoots. But since a limiter is essentially a really fast compressor, and nowadays with a “look ahead” function, driving audio levels into the limiter gets you loudness too. However, itʼs a different kind of loudness compared to what comes from multiband compression. While a compressor achieves loudness via smoothness, the limiter does it through aggressively attacking peaks in the audio resulting in a more active, energetic sound.

COMPRESSION & LIMITING FOR LOUDNESS

Now, how do we choose how much compression or limiting to use? Just as we did with AGC, we need to choose ahead of time what kind of sound we want for the given format: Loud and smooth, or loud and energetic. This is important because we can very easily end up with too much compression and too


much limiting competing with each other. In other words you end up with a “smooth” “energetic” sound which doesnʼt work. Itʼs far better to let one or the other take a back seat so that the character of your format shines through. Listeners may not be able to articulate exactly why your station sounds as sonically congruent as it does but they will feel it subconsciously because youʼve put in the work thinking it through.

The “seesaw” graphics below are examples of balancing compression vs. limiting and how that relates to the sound of typical station formats. In the first example, heavier compression and less limiting is used to achieve a loud yet smooth sound appropriate for formats such as Jazz, Country, Variety, Talk and Easy Listening. In the second example, heavier limiting and less compression is used yielding a loud yet energetic sound appropriate for Dance, Rock, Urban and Pop formats.

COMPOSITE CLIPPING

The very last processing stage of a broadcast audio processor is always the composite clipper, and while AGC, compression, and limiting treat dynamics in increasing order of speed, the composite clipper works instantaneously. It slices off any audio peaks that might make it through the limiter stage and final stereo generator stages to protect the transmitter from overshoots. However, because it actually flattens out the peaks of those waveforms, manually dialing-in more clipping gives you distortion and extra harmonics. Obviously a lot of distortion is not good, but in small amounts, a bit of composite clipping can add that extra bit of “Edge” and loudness to your sound. Composite clipping use should be subtle, so if you can hear it all the time, or on vocals and sustained notes, itʼs too much!


PRESET SWITCHING & METERING

One of the great luxuries of modern digital audio processors is that we can create, save, and compare presets on the fly. This is immensely helpful because it allows us to make incremental changes over time and then A/B test to see if weʼve truly made improvements. The human ear adjusts very quickly to what itʼs hearing, and it can be difficult to remain objective when making multiple adjustments. So save off custom presets, wait a bit and then go back and compare.

Finally, fast and accurate modulation metering is essential. Total modulation metering from a good Mod Monitor should always be used when adjusting compression and limiter parameters because we can actually gauge with our eyes (as well as our ears) how much dynamic range is being preserved. If the modulation monitor is pegged at 100% and thereʼs very little movement, chances are thereʼs too much compression or limiting going on.

Finally, getting broadcast audio processing right can be a complex and often times overwhelming proposition. But with some basic understanding of the building blocks, concepts of loudness perception, prep work on your formatʼs characteristics, good metering and preset switching, we can make processing adjustments with confidence!


Getting The Most From Your Audio Processor

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